A solid high-polymer-type fuel cell functioning as a fuel cell stack is composed by laminating a plurality of units, each unit being a laminated body of separators laminated on both sides of a flat membrane electrode assembly (MEA). The membrane electrode assembly is a three-layer structure consisting of a pair of gas diffusion electrodes forming a positive electrode (cathode) and a negative electrode (anode), and an electrolyte membrane of ion exchange resin inserted between them. The gas diffusion electrodes are formed at the outside of an electrode catalyst layer contacting the electrolyte membrane. The separators are laminated to contact the gas diffusion electrodes of the membrane electrode assembly, and a gas passage for passing gas and a refrigerant passage are formed between the gas diffusion electrodes. In this fuel cell, for example, hydrogen gas flows as fuel in the gas passage facing the anode-side gas diffusion electrode, and oxidizing gas such as oxygen or air flows in the gas passage facing the cathode-side gas diffusion electrode, and thereby an electrochemical reaction takes place and electricity is generated.
The separator has a function of supplying electrons generated by catalytic reaction of hydrogen gas at the anode side to an external circuit, and also of supplying electrons from the external circuit to the cathode side. The separator is made of conductive material such as graphite or metal; in particular, metal is advantageous because the mechanical strength is excellent and separator can be designed in a compact and lightweight structure by using thin plates. As the metallic separator, a stainless steel thin plate having nonmetallic conductive inclusions forming conductive paths projecting from the surface is preferably used. In the producing method for such a separator, conductive inclusions are projected form the surface of stainless steel having conductive inclusions in the metal structure to obtain a separator material plate, and this separator material plate is press-formed into an undulating surface, and the grooves formed on the front and back surfaces are used as the gas passage and the refrigerant passage, respectively. The process of projecting the conductive inclusions includes chemical etching, electrolytic etching, sand blasting, and others for removing the surface of the base material.
FIG. 1A schematically shows the surface of the separator material plate obtained by projection processing of the conductive inclusions. In the drawing, reference numeral 10 is a base material, and 20 indicates conductive inclusions. When this separator material plate is press-formed, the conductive inclusions 20 projecting from the surface of the base material 10 penetrate into the base material 10, as shown in FIG. 1B, but in this state gaps 30 may be formed at the interfaces of the surface of the base material 10 and the conductive inclusions 20. These gaps 30 cause pitting or crevice corrosion as the power generation by the fuel cell preceds, and the conductive inclusions may drop out, and the contact resistance to the membrane electrode assembly may increase, thereby leading to a drop in the power generation performance.